"You know, some time down the road. Like anything, I would say that to all of our games. So that's not a promise of when or where."

Valve's alleged inability to count to three has formed the basis of many a joke across online communities, following the studio's reluctance to release threequels to any of its popular series'.

Players have been waiting for years for news on Half-Life 3 after Half-Life 2: Episode Three failed to materialise.

Following the release of Left 4 Dead 2 in 2009, Valve released Portal 2 last year to critical acclaim. It's currently putting the finishing touches to MOBA-title Dota 2, and continues to provide support for Team Fortress 2.

But the studio may finally move on to a third-numbered game with Left 4 Dead.

"I really love working on that series," Faliszek continued. "It's one of my favourite series. It's just so much fun with the characters and the world. I love zombies and I love horror. So, yeah."